virsysinfo: s/system/sysdef/

virsysinfo: s/system/sysdef/

A variable can't be named system, obviously. Well, it can if the
compiler is new enough to distinguish a variable named system and a
function call system(). And some older systems, don't have wise
compiler.

  CC     util/libvirt_util_la-virsysinfo.lo
cc1: warnings being treated as errors
../../src/util/virsysinfo.c: In function 'virSysinfoParseSystem':
../../src/util/virsysinfo.c:649: error: declaration of 'system' shadows a global declaration [-Wshadow]
/usr/include/stdlib.h:717: error: shadowed declaration is here [-Wshadow]
make[3]: *** [util/libvirt_util_la-virsysinfo.lo] Error 1

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>